1. Field of the Invention
This invention relates to lighting devices and systems, and in particular, it relates to high brightness light sources.
2. Description of the Related Art
Light sources are used in a wide variety of application, including image projection such as rear projection TV (RPTV) or front projector, headlights or illumination lights for transportation vehicles such as automobiles, motorcycles, boats and airplanes, etc. One import requirement for a light source is to provide high brightness and high power output at the same time. Currently, light sources for these and other applications are still dominated by traditional light sources such as high-pressure mercury lamps, Xenon lamps or metal halide lamps. However, the arc lamps have technical limitations in many applications: relatively short lifetime, difficulty to control and maintain its color, un-stability especially when operating in a pulsed mode. For many applications, especially in an environment where heat generation is undesirable or when electricity is unavailable, light from the light source needs to be coupled into optical fibers or waveguide and directed to where illumination is required. However, the coupling from the arc lamp into fiber can be costly, bulky, inefficient and unstable due to, e.g., the change of discharge arc itself from time to time. In many cases, the arc lamp also potentially interferes with other components in a system.
Solid state light sources, especially light emitting diodes (LEDs), exhibit longer lifetimes, lower power consumption, manageable wavelengths and other benefits in comparison with the above and other traditional light sources. Therefore, these solid-state light sources increasingly become the alternative or even preferred choice of light sources for a variety of applications. However, there are many performance issues that need to be improved for LEDs so that their applications can be broadened. Currently, two potential solutions can be explored to achieve high brightness and high power LED light sources. The first one is to further improve individual LED chip performance by increase chip dimension and improve its quality. However, this approach is limited by the total output of one individual chip, currently in tens of lumen level in the visible wavelength range. Going to larger area chips and higher driving currents can increase the total output but will compromise the device lifetime and brightness. The chip uniformity and thermal dissipation of large LED chip are serious limitation for this approach. The second approach is to package many LED chips together in an array structure to obtain high total output, up to hundreds even thousands lumens currently. The brightness of light directly from an LED array is significantly lower than that of single LED since the array brightness is limited by the relatively low package density of LEDs in the array. The major challenge of high-density LED packaging is the thermal management of the high power operation of LEDs since the LED interferes each other thermally if they are too close to each other. Due to the reasons discussed above, when the application demanding high brightness and high power at the same time, the traditional light source such as high pressure mercury lamp or metal halide lamp are still the choice of applications.